Conventionally, as described in Japanese Laid-Open Patent Publication No. 2002-364409, for example, a fuel injection device capable of switching fuel injection modes is known. More specifically, the fuel injection device is employed in an internal combustion engine having a direct injector injecting fuel directly into a combustion chamber and an intake passage injector injecting fuel into an intake passage. The fuel injection device switches the fuel injection modes through selective actuation of the direct injector or the intake passage injector.
Since the pressure in the combustion chamber is relatively high, the fuel injection pressure (the fuel pressure) of the direct injector, which injects fuel into the combustion chamber, is normally set to a relatively high level, as compared to that of the intake passage injector. In this manner, the direct injector injects the high-pressure fuel into the combustion chamber, such that vaporization of the fuel is promoted in the combustion chamber.
However, the fuel injection device may lead to the following problem. That is, when the fuel is pressurized to a relatively high level in a fuel pressurizing system for the direct injector, a noise may be produced by such operation in the fuel pressurizing system. To suppress the noise, a target pressure for the fuel pressurization may be lowered if the engine is idling. However, this may cause a delay in the fuel pressurization when the engine operation is switched from an idling state to, for example, a high-load state, at a later stage. In this case, the fuel is pressurized to only an insufficient level or cannot reach a level required for the high-load engine operation.
If the actual fuel pressure cannot achieve the required level, as described above, the fuel injected by the direct injector is not adequately atomized. This may hamper the fuel combustion, lowers the engine torque, and aggravates exhaust properties, in disadvantageous manners.